President Obama is asking Congress to approve $500 million to train and equip moderate Syrian rebels struggling to oust President Bashar Assad after more than three years of civil war.

The opposition rebels would be vetted before being given assistance, according to a statement from National Security Council spokeswoman Caitlin Hayden. Hayden said the funds would "help defend the Syrian people, stabilize areas under opposition control, facilitate the provision of essential services, counter terrorist threats, and promote conditions for a negotiation settlement."

The request marks the first time the Obama administration would be overtly supplying weapons to the Syrian rebel forces in a program run by the Pentagon under the military budget, according to U.S. official. It has the advantage of enabling the U.S. government to send more sophisticated weapons, and allows it the ability to discuss the program publicly to allies and congressional critics.

But sources told NBC News the administration has been covertly arming the rebels for the last year through the CIA, providing opposition forces with less sophisticated weaponry in a program that had been advocated years earlier by both former General David Petraeus and former Secretary of State Hillary Clinton.

Obama hinted at the increased support for the opposition forces during a commencement address at West Point last month. "In helping those who fight for the right of all Syrians to choose their own future, we also push back against the growing number of extremists who find safe haven in the chaos," Obama told the graduating cadets.